After tune up blues-update!

[Mad Gringo]

If you have read my previous thread, and made comments I appreciate it. Two cylinders, middle two next to each other lost compression. Ran a compression leak test and semi confirmed bad head gasket. Head removed and it appears it might be the case. But...theres always a damn but. Those two cylinder's intake valves are looser than the others, and appear to be taller than the others. Any ideas?

[a64pilot]

Want do you mean by the valves are looser? And what do you mean by taller?
Take the head to a shop and have it disassembled, only a few minutes work, then you will know. Forget the questions, the risk does not outweigh taking it apart and being certain nothing is wrong.
See, I don’t think the spark plug tip damaged the head gasket, I think the engine was timed way too far advanced, resulting in stupid high pressures which broke off the plug tip and blew the gasket.
If the spark is too far advanced, the peak cylinder pressure will occur just before TDC, the inertia of the engine will force the piston to continue and pressures will go through the roof, something has to give, sometimes its the piston head, cause temps go crazy high too.

Al lot of people call this detonation, which it is not, however the results are very similar.
Detonation is a spontaneous, instantaneous ignition of the entire fuel charge without it being initiated by the spark plug.

[Mad Gringo]

The springs are looser than the others. The top of the valves are a eighth to quarter of inch taller/higher than others. Having both valve job both sides - heads are at shop now.

[jimbunyard]

Very curious now. Can you tell us what procedures were carried out in the original 'tune up'?


Were there no symptoms that led you to being convinced to do a tune up?

[a64pilot]

Originally Posted by Mad Gringo

The springs are looser than the others. The top of the valves are a eighth to quarter of inch taller/higher than others. Having both valve job both sides - heads are at shop now.

Valves are either recessed which can happen from being way overheated, or they have been stretched somehow.
Machine shop should be able to tell you want the deal is though.
Often times valves are actually two piece with the head welded to the stem, and that is where they fail sometimes and then it’s called dropping a valve, which is catastrophic to a motor.
But all this is speculation, machine shop will be the ones to tell you what happened.
Let us know, my curiosity is peaked

[Mad Gringo]

Slight miss in starboard engine. I have owned the boat 5 years and had not had it tuned, thought I should be proactive. Never again! New everything then all new everything replaced with newer everything. After first attempt I lost throttle response in port engine. After second attempt I had same problem and boat would not idle correctly. Bad plugs and wires was told. Now port engine disassembled and heads are in shop for valve job. Bad head gasket caused by badly timed job, I think! Only time and money will tell.

[alaskaflyfish]

Originally Posted by a64pilot

Want do you mean by the valves are looser? And what do you mean by taller?
Take the head to a shop and have it disassembled, only a few minutes work, then you will know. Forget the questions, the risk does not outweigh taking it apart and being certain nothing is wrong.
See, I don’t think the spark plug tip damaged the head gasket, I think the engine was timed way too far advanced, resulting in stupid high pressures which broke off the plug tip and blew the gasket.
If the spark is too far advanced, the peak cylinder pressure will occur just before TDC, the inertia of the engine will force the piston to continue and pressures will go through the roof, something has to give, sometimes its the piston head, cause temps go crazy high too.

Al lot of people call this detonation, which it is not, however the results are very similar.
Detonation is a spontaneous, instantaneous ignition of the entire fuel charge without it being initiated by the spark plug.

I think your explanation is spot on. Too much internal pressure as a result of advanced timing. I've seen the same thing happen with turbo-supercharged engines.

[Heezea]

I know it's easy to think the mechanic made a mistake. I know I would intuitively have the same thought. However, after working on my old truck I found that if I fixed one thing then something else would often break. I came to think of it like a chain. Imagine a load on the chain breaks one of the links so you replace the broken link. Now the same load is put back on the chain and if a second weak link was further down the chain, it would now break. It didn't break before because the first link failed and so the load never got to the second link.

In your case, it could be that the weakened spark wasn't providing as much pressure in the cylinder. Once replaced, the stronger spark may have provided the proper cylinder pressure. Since the head gasket was also weak, it could stand up to the weaker pressure but not to the standard pressure and so it failed.

[alaskaflyfish]

The theory of multiplicity (meaning one event leads to another) is possible here and every mechanic has to deal with the next weak link of causation. My own mother used to swear up and down that the mechanic who changed her tire caused the muffler to fail. However, in this case the OP stated that the motors ran smooth and produced power prior the the tune-up. That could not have been true if there were two dead cylinders prior to tune-up. It would hardly run. Two elongated valves (post timing) means that the valves were pounding in their seats due to internal pressures. Broken spark plug tip would mean the same (unless the plug was dropped-cracked prior to install). Then the spark plug tip could cause havoc in multiple cylinders (I've seen it happen) usually shown as peened cylinder head and broken exhaust valve tulip which the OP did not mention, but then explain the blown head gasket? A preponderance of evidence given suggest aggressively advanced timing.

[Perna]

The valves would appear 'taller' due to valve and valve seat wear. If the valve clearances are not reset in time, or they are set too tight during an overhaul then it can become a vicious circle. Valve no longer seals properly, combustion gases pass to the ports and the whole thing gets very very hot, causing more wear, head distortion and head gasket damage. In the end compression is lost on those cylinders.

First sign would probably be a misfire on those cylinders.

[Cheechako]

Hard to say but have that head rebuilt. If the valve seats were ground into the head they will look taller. If the valve guides are shot whey will be loose. If the valves are overheated and pulling into the head they will look taller. head gasket gone too? Time for complete head rebuild!

[Mike Banks]

A quarter of an inch? Sounds like missing valve-seat inserts. Perhaps the wrong valves used, and loose valve springs could mean wrong springs or springs that have been grossly overheated or were el-cheapo knock-offs never properly tempered in the first place.

I would suspect missing valve seat inserts. That would account for lack of compression and slack springs as well as "longer" valves.

[redsky49]

But he is saying the bad valves are the intakes, not the exhausts. I have seen out-of-adjustment exhaust valves burn and recede. Are intakes subject to this in the same way? Not cooled by the intake charge?

[a64pilot]

Machine shop that takes it apart, should have some answers